Tested: vortex generators (AirTabs) on a Toyota Corolla

[chuckm]

Well, it is just a bump. Best I can do with that flow calculator.

[MetroMPG]

Video:

YouTube - Tuft testing vortex generators - Toyota Corolla aerodynamics

[azraelswrd]

Nice summary vid. I agree with the theory that the advantages of improved air flow are being cancelled out by the increased frontal area from the tabs themselves. This makes sense why the tabs were originally designed for semi's and rigs since their frontal area is already so large that the additional FA due to drag vortices from the tabs would not be as significant as the gains from airflow.

Neil's own tests with the tabs on his xA echo your results with no significant improvement (with evidence of actual decreased MPG)

[garys_1k]

Originally Posted by MetroMPG

I'll admit my jaw dropped when I saw the obvious change in flow shown by the tufts!

But I had to keep reminding myself: vortex generators themselves create drag. If the size, style & placement isn't optimized, you can end up with a net increase in drag (or zero sum gain), even while you apparently improve downstream flow.

We could just as easily stick a wind deflector at the trailing edge of the roof to force air down the rear window and tidy up the tufts. But we'd end up with turbulence aft of the deflector negating the gain of the improved flow on the glass/trunk lid.

Thanks so much for doing the grunt work and testing these things! You answered a LOT of questions we had about them.

[MetroMPG]

So let's keep digging...

To calculate the projected area of an AirTab, I took this picture on maximum zoom (and kept it in the center of the viewfinder to minimize lens/perspective distortion):



Traced its outline in Photoshop and made it into a 2 colour image:



Using the Image > Histogram... feature there are 9,480 black pixels in this image. The 61 pixel height of the black corresponds to 26 mm (1 inch) height of the actual AirTab. So 1 mm = 2.346 px; and 1 mm2 = 5.504 px.

So the projected area of the AirTab is 9480 px/5.504 px per mm2 = 1722.25 mm2, or 17.2 cm2 or 2.67 square inches.

Times 8 AirTabs (though Donee pointed out the outer 2 probably arent needed): 137.8 cm2 or 21.4 square inches!

Taken together, that's pretty much the projected area of a door mirror! OK, a small mirror.

Just eyeballing it, two delta-wing VG's set at the angle Mitsu used on the EVO are probably 1/3 the projected area of one (double vortex producing) AirTab.

[roflwaffle]

Ack, wrong place! I think... Comparing my Grandpa's Corolla (.3) to his old Camry (.34) the angle from the edge of the trunk to the roof isn't as great and the C pillars curve in to help w/ pressure drag at that part of the vehicle.

I could be wrong, but AFAIK the whole point of VGs was wake infill. Not nearly as great as a better shape, but beter than a kick in the head. I just don't think it's needed on top of the trunk (by putting 'em on the roof). Any chance you could test 'em on top on the trunk near the abrupt drop back there?

[Z man]

Do you think the wake of the xB is much different than the Corolla (Steep rear angle), and would VGs have a different impact? I would appreciate any improvement I can get with my horrible Cd!

[trebuchet03]

Originally Posted by roflwaffle

I could be wrong, but AFAIK the whole point of VGs was wake infill.

Filling in wake is something that doesn't have any research behind it (at least, I haven't been able to find anything despite searching for it). The reason for them, and the decades of research experience revolves around, is to serve as a band-aid for low energy flow. Airtab, awhile back as I recall, claimed this wake fill phenomena - but it seems to have been removed from their site...

[roflwaffle]

Seems pretty common in motorsports. It usually isn't called wake infill, but reducing the pressure drag via aero band aids isn't new AFAIK... On pg. 18 there's something illustrating the small difference in flow separation, so I'm guessing the better bet would be on top of the trunk near the flat back. Here's some more stuff that may or may not be accurate.

[Xringer]

"The Frame Extension device is designed to not interfere with the operation and loading requirements of a rear door vehicle.
The two side panels and top panel of the invention are inset from the outer edge of the frame less than 3 inches whereas the bottom panel is structurally designed to support loading and off loading operations. Properly sized, this technology will increase fuel economy up to 6%."

6% would be real nice! Looking at the pic, it's a sorta Boat-Tail that one could add to a squared off SUV with some tape and Coroplas..

------
But this undercarriage in-flow stuff:

"Undercarriage drag results from high-energy flow entering the underside of the vehicle and impinging upon the various structures and components. The Mini-Skirt invention redirects and captures the majority of the undercarriage in-flow through the use of a vortex induced upwash field. Compared to traditional single panel skirts the novel inner and outer skirt surfaces of the Mini-Skirt invention allow for the blockage of a greater amount of undercarriage in-flow for a given skirt drop length.

The outer skirt surface of the Mini-Skirt has an aerodynamically sharp lower edge that is used to generate the controlling vortex that induces the upwash field. The induced upwash is then captured and redirected by the inner skirt surface. When installed in accordance with design requirements the Mini-Skirt technology will increase fuel economy up to 6%. "

This has me more than a little confused. Two side skirts? With one that is inset behind the outer.. Where it redirects upwash air into downwash?
And this downwash air pressure blocks impinging air better than a "traditional single panel skirt"??

I need some diagrams..

Found some:
http://www.solusinc.com/miniskirt.html